A senior U.S. senator is asking airlines about their data privacy practices, saying he’s concerned about what information the companies are collecting and sharing with third parties.

Some consumer advocates have raised concerns that airline privacy policies “can contain substantial caveats and that it is difficult for consumers to learn what information airlines and others in the travel sector are collecting, keeping, and sharing about them,” Senator John “Jay” Rockefeller, a West Virginia Democrat, wrote in a letter to 10 U.S. airlines Monday.

The airlines receiving the letters included United Airlines, Delta Air Lines, American Airlines and Southwest Airlines. Airlines contacted about Rockefeller’s letter didn’t immediately respond to requests for comments.

A spokeswoman for Rockefeller, chairman of the Senate Commerce, Science and Transportation Committee, didn’t point to specific complaints about airline privacy policies, but noted that he has focused on raising consumer awareness about the personal information they provide online. This year, Rockefeller has introduced theData Security and Breach Notification Act, which would create a federal standard for companies to safeguard the personal information they hold and to notify consumers if their systems are breached.

“Data collected during ticket purchase can include a passenger’s name, credit card numbers, date of birth, addresses, travel destinations, and travel companions, among other information,” Rockefeller wrote in the letter. ”No comprehensive federal privacy law currently applies to the collection, use, and disclosure of consumer travel information.”

Some privacy groups have raised concerns about airline privacy in recent years, but the focus has been more on airline security measures after the 9/11 attacks on the U.S. than on airlines collecting and sharing personal information.

Much of Rockefeller’s letter covers add-on fees that airlines charge customers for carry-on luggage, priority seating and other services. But the letter also asks the airlines what personal information they collect, how long they keep it and what privacy and security protections they have in place.

The letter also asks the airlines whether they share the personal information with third parties and the reason for sharing it.